The present invention relates to a solid electrolytic capacitor and a method for producing the same. More specifically, the invention relates to a solid electrolytic capacitor with modifications for the purpose of improving the capacitance occurrence ratio by reducing the equal serial resistance (referred to as ESR hereinafter) and for making the resulting capacitor into a small size.
Electrolytic capacitors using valve action metals such as tantalum or aluminum, are widely used, because such electrolytic capacitors can be made into small sizes and can also generate large capacitance, owing to the enlargement of the area of the dielectric material via the shaping of the valve action metal as the counter electrode on the anode side into sintered material or etched foil. Particularly, solid electrolytic capacitors using solid electrolytes as the electrolytes have characteristic properties such as facility of chip formation and suitability for surface mounting, in addition to small type, large capacitance and low ESR. Therefore, such solid electrolytic capacitors are indispensable for the production of small-sized, highly functional electric devices at low cost.
For such type of small-sized solid electrolytic capacitor for use for large capacitance, generally, the capacitor element therein is produced by winding together an anode foil and a cathode foil via a separator, both the foils comprising a valve action metal such as aluminum, impregnating the capacitor element with a driving electrolytic solution and placing the capacitor element in a case made of metals such as aluminum or a case made of synthetic resins such that the resulting capacitor has a sealed structure. Further, aluminum, tantalum, niob and titanium are used as the anode material, while metals of the same kind as those for the anode material are used as the cathode material.
So as to increase the capacitance of electrolytic capacitor, importantly, the capacitance of the cathode material as well as the capacitance of the anode material should be improved. The capacitance of each of the electrodes of electrolytic capacitor is defined by the type and thickness of an insulation film formed on the electrode surface and the surface area of the electrode. Given that the dielectric constant of insulation film is ∈; the thickness of insulation film is t; and the surface area of electrode is A, the capacitance C is expressed by the following equation.
C=∈(A/t)
So as to increase the capacitance, as shown by the equation, it is effective to enlarge the electrode surface area, select an insulation film material with a high dielectric constant and produce a thin insulation film.
Among them, simple use of large electrode so as to enlarge the electrode surface area is not preferable, because such use only makes the resulting electrolytic capacitor into a large size. Thus, the surface of aluminum foil as a fundamental material for electrodes has traditionally been etching processed to form recesses and protrusions thereon to substantially enlarge the surface area.
Additionally, Japanese Patent Laid-open No. 167009/1984 discloses a cathode material with a metal film formed on the surface of the base material by utilizing the metal deposition technique, as an alternative of the etching process. According to the technique, film-forming conditions should be selected, so as to enlarge the surface area by forming microfine recesses and protrusions on the film surface to yield a large capacitance. Additionally, metals exerting high dielectric constants in the form of oxides thereof, such as Ti, can increase the dielectric constant of the insulation film formed on the surface of the cathode material, leading to a larger capacitance.
Furthermore, Japanese Patent Laid-open No. 150825/1991 previously filed by the present applicant discloses a technique for forming a deposition layer comprising titanium nitride on the surface of high-purity aluminum used as a cathode electrode by cathode arc deposition process, so as to increase the capacitance value of the cathode side, in the light of the finding that the capacitance of electrolytic capacitor is the composite capacitance based on the capacitance of the anode side and the capacitance of the cathode side in serial connection.
Problems to be Solved
However, solid electrolytic capacitors using the cathode foils formed by the conventional techniques described above have the following drawbacks. More specifically, the surface of aluminum foil as a fundamental material for electrodes of conventional solid electrolytic capacitors is etching processed, so as to increase the capacitance of the electrolytic capacitors. When etching is processed too excessively, the solubilization of the surface of the aluminum foil concurrently progresses, which adversely blocks the increase of the surface enlargement ratio. On the basis of such reason, the increase of the capacitance of electrode material by etching technique is limited.
Additionally, the technique for forming a deposition layer comprising titanium nitride on the surface of cathode foil is also problematic. More specifically, manganese dioxide formed via thermal decomposition of manganese nitrate has mainly been used as the solid electrolyte of conventional solid electrolytic capacitors. During the process of forming manganese dioxide, however, thermal treatment at 200 to 300 xc2x0C. should be carried out several times. Therefore, oxide film is formed on the surface of the metal nitride film formed on the surface of the cathode foil, which causes the reduction of the capacitance of the cathode foil, leading to the reduction of the capacitance of the electrolytic capacitor. Furthermore, ESR reduction is limited, because manganese dioxide has a relatively high electric conductivity.
Objects of the Invention
The present invention has been proposed so as to overcome the problems of the conventional techniques. It is a first object of the invention to provide a solid electrolytic capacitor with improved capacitance occurrence ratio and a method for producing the same. It is a second object of the invention to provide a solid electrolytic capacitor not only with improved capacitance occurrence ratio but also with reduced ESR, and a method for producing the same.